446 



ENGINEERING ON THE FARM 



telephones. It was found that a call bell of a type which will 

 respond to an alternating or pulsating current of relatively 

 low frequency was admirably suited to the purpose, and such 

 bells are now in quite general use in telephone practice. 

 To provide the necessary alternating current for oper- 

 ating the bells and the signals at the central offices in local 



CONTACT 

 SPRING 

 ASSEMBLY 



END MAGNETS 



GEAR 



AND SLEEVE 



MAIN SHAFT SPRING 



After Western Electric Co. 



Fig. 340. Magneto or generator as used in local battery stations 



battery systems, a small dynamo electric machine similar 

 to those which are in common use in electric-light and power 

 plants, except that permanent magnets are used for the fields, 

 was found to be effective. This device is operated by a 

 crank protruding through the right side of the instrument, 

 called the magneto. 



A form of switching device was needed for changing the 

 connections of the line conductors from the signaling device 

 or bell to the talking devices. The hook switch, which is 



